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Temperature differences create electricity

30 November 2011

Temperature differences can be used to create electricity, or so-called thermoelectricity, according to scientists at the Centre for Materials Science and Nanotechnology at the University of Oslo (UiO) and the Foundation for Scientific and Industrial Research (SINTEF) in Norway.

By Kari Williamson

Thermoelectric materials are already in use, such as for powering space probes using plutonium – exploiting its high temperature compared to the cold of space. Thermoelectric materials are also used in cooler bags using lead and tellurium. However, these solutions are all toxic.

"We want to replace them with inexpensive and readily available substances. Moreover, there is not enough tellurium to equip all of the cars in the world," says Ole Martin Løvvik, Associate Professor in the Department of Physics at UiO and Senior Scientist at SINTEF.

A team of scientists led by Professor Johan Taftø, is therefore now searching for pollution-free, inexpensive materials that can recover 15% of all energy losses, compared to today's 10%.

Nanotechnology solution

"I think we will manage to solve this problem with nanotechnology. The technology is simple and flexible and is almost too good to be true. In the long run, the technology can utilise all heat sources, such as solar energy and geothermal energy. The only limits are in our imagination," Løvvik says.

The new technology will initially be put to use in thermoelectric generators in cars. Several major automobile manufacturers are said to be interested, and Løvvik and his colleagues are currently discussing possibilities with General Motors.

"Modern cars need a lot of electricity. By covering the exhaust system with thermoelectric plates, the heat from the exhaust system can increase the car's efficiency by almost 10% at a single stroke. If we succeed, this will be a revolution in the modern automotive industry.”

Silent fridge

"In the future, refrigerators can be soundless and built into cabinets without any movable parts and with the possibility of maintaining different temperatures in each compartment,” Løvvik adds.

In order to extract as much energy as possible, the temperature difference should be as large as possible, although there is an upper limit to what materials can handle.

The ideal thermoelectric material

A good thermoelectric material ought to be a semi-conductor with very special properties: Its thermal resistance must be as high as possible at the same time as current must flow through it easily.

"This is not a simple combination, and it may even sound like a self-contradiction. The best solution is to create small structures that reflect the heat waves at the same time as the current is not reflected.”

When a material becomes hot, the atoms vibrate. The hotter it becomes, the greater the vibrations, and when an atom vibrates, it will also affect the vibration of the adjacent atom. When these vibrations spread through the material, they are called heat waves. If barriers are created in the material so that some atoms vibrate at different frequencies from their adjacent atoms, the heat will not be so easily dissipated.

"Moreover, the atomic barrier must be created in such a way that it does not prevent the electric current from flowing through it,” Løvve explains.

The atomic barriers are introduced densely in the special semi-conductors.

"We have achieved this by using a completely new 'mill'. Just as the miller grinds grain, the scientists will grind down semi-conductors to nano-sized grains. They will do that by cooling them down with liquid nitrogen to minus 196°. That makes the material more brittle, less sticky and easier to crush. It is important to grind down the grains as small as possible. Afterwards the grains are glued back together again, and in this way the barriers are created.

"The small irregularities in the barriers reflect the heat waves," Løvvik says.

"We have now discovered new nano-cavities in the materials and learned more about how they reflect heat waves.”

Renaissance for cobalt.

The scientists are now searching for the next generation of thermoelectric materials. They have tested the cobalt arsenide mineral, skutterudite, which may be found at Skutterud at Blåfarveværket in Modum, Norway. "It was just recently discovered that skutterudite may have atoms located in small nano-cavities. These cavities act as barriers to heat dissipation," concludes Løvvik.

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